The present invention relates to a sheet feeding apparatus arranged to feed a sheet in forward and backward directions, more particularly to a sheet feeding apparatus capable of controlling feeding operation so as to be executed without a feeding error due to a backlash of gears for driving a sheet to be fed.
FIG. 1 is a sectional view of a printer as a sheet feeding apparatus.
A printer frame 1 rotatably supports a platen 2 around a platen shaft 2A. Below the platen 2, a plurality of pinch rollers (two rollers 5, 5 being shown in FIG. 1) for nipping and sending a cut sheet 3 are disposed along with the platen 2. In addition, a paper bail 8 movable toward and away from the platen 2 is also disposed against the platen 2. Further, a printing head 6 is mounted on a well-known carriage movable along the platen 2. A ribbon cassette 7 housing a printing ribbon is detachablly mounted on the carriage and is moved along the platen 2.
Over the platen 2, a sheet supply/receiving device 21 including a hopper 25 for stacking the cut sheets 3 in an overlapped state is disposed, the upper surface of the hopper 25 being inclined to the printer. The sheet supply/receiving device has a frame 22 comprising a pair of side frame members 22A and a front wall panel 22B extending between and supported by the side frame member 22A. The side frame members 22A have a pair of respective connector arms 23 engageable with the opposite ends of the platen shaft 2A. A pair of sheet guides 23A, 23B that are spaced from each other is disposed between and supported by the connector 23. On the frame 22 of the hopper 25, a drive roller 26 for separating the top cut sheet 3 from the other sheets and for sending it toward the platen 2 and a pair of rollers 27 and 28 disposed midway between the drive roller 26 and the platen 2 in the sheet sending direction for sending the sheet are disposed, the rollers 27 and 28 being contacted each other, the cut sheet 3 being passed between the rollers 27 and 28. The rollers 26, 27 and 28 are respectively mounted on shafts 26A, 27A and 28A around which the rollers are rotated.
FIG. 2 is a view outlining a meshing state between a platen gear 50 secured to the platen shaft 2a and a motor gear 51 connected to a shaft 10A of a first motor "MO" which is a drive source for driving the platen 2. The first motor "MO" is a stepping motor.
FIGS. 3A and 3B are enlarged views showing the meshing state of both the gears. FIG. 3A is a view showing the meshing state of the platen gear 50 and the motor gear 51 when the cut sheet 3 is started to be forwardly fed from the left to the right of the drawing as indicated by an arrow "A". On the other hand, FIG. 3B is a view showing the meshing state of the platen gear 50 and the motor gear 51 when the cut sheet 3 is started to be backwardly fed from the right to the left of the drawing as indicated by an arrow "B". As apparently shown in the drawings, in a typical gear transmission mechanism, by considering various errors such as production error, mounting error, and thermal expansion which may occur in the gears, a proper clearance, or backlash, "t" is provided between the gears which are meshed. Thus, when the motor gear 51 is rotated from the forward direction to the backward direction or vice versa, the rotation of the platen gear 50 involves a delay corresponding to the backlash "t". Consequently, even if the motor gear 51 is rotated for a specific amount, the platen gear 50 insufficiently rotates. Thus, the sheet feeding operation cannot be precisely accomplished. For example, when characters are over-stuck, the printing positions deviate, resulting in degradation of the printing quality. In a prior art disclosed in Japanese Patent Provisional Publication SHO 60-31983, when the sheet is forwardly fed, the drive section is forwardly rotated for a specific feed amount; when the sheet is backwardly fed, the drive section is excessively and backwardly rotated for a specific amount which is greater than the specific amount and then the drive section is forwardly rotated for the extra amount so as to prevent the feed error due to the backlash.
However, in the typical printer as illustrated in FIG. 1, when the sheet is forwardly fed, the platen 2 is driven in synchronization with the rollers 27 and 28. Conversely, when the sheet is backwardly fed, the platen 2 is backwardly rotated, but the rollers 27 and 28 and the drive roller 26 are not rotated because the subsequent stacked sheet is not backwardly fed. Thus, when the sheet is backwardly fed, since the rollers 27 and 28 are in non-rotatable state, they cannot send the cut sheet 3. Consequently, as shown in FIG. 4 of an outlined view of the sending state of the sheet, the cut sheet 3 is sagged midway between the platen 2 and the rollers 27 and 28.
In the following manner, the platen 2 is controlled so as to prevent the feed error due to the backlash with the manner described in the above prior art. First, the motor gear 51 is driven by the first motor "MO" for the specific extra amount which is greater than the specific retreating amount and the platen 2 is reversely rotated. At that time, the gears are meshed as shown in FIG. 3B. Then, likewise, when the motor gear 51 is driven for the extra amount and the platen 2 is forwardly rotated, the gears are meshed as shown in FIG. 3A.
However, when the cut sheet 3 is backwardly fed as described above, the cut sheet 3 is sagged between both the rollers. A restoring force is applied to the cut sheet 3, thereby causing the platen 2 to be forwardly rotated from the state shown in FIG. 3A.
Thus, the platen 2 is advanced for the amount of the backlash "t". If the platen 2 is forwardly rotated for the extra amount, the cut sheet 3 is excessively advanced for the amount of the backlash "t" of the gears. Consequently, the cut sheet 3 cannot be precisely sent to the specified position, resulting in a deviation of the printing positions when an overstruck printing operation is executed.